The present invention relates generally to semiconductor device manufacturing and, more particularly, to a graphene based, three-dimensional (3D) integrated circuit device.
Graphene refers to a two-dimensional planar sheet of carbon atoms arranged in a hexagonal benzene-ring structure. A free-standing graphene structure is theoretically stable only in a two-dimensional space, which implies that a truly planar graphene structure does not exist in a three-dimensional space, being unstable with respect to formation of curved structures such as soot, fullerenes, nanotubes or buckled two dimensional structures. However, a two-dimensional graphene structure may be stable when supported on a substrate, for example, on the surface of a silicon carbide (SiC) crystal. Free standing graphene films have also been produced, but they may not have the idealized flat geometry.
Structurally, graphene has hybrid orbitals formed by sp2 hybridization. In the sp2 hybridization, the 2s orbital and two of the three 2p orbitals mix to form three sp2 orbitals. The one remaining p-orbital forms a pi (π)-bond between the carbon atoms. Similar to the structure of benzene, the structure of graphene has a conjugated ring of the p-orbitals, i.e., the graphene structure is aromatic. Unlike other allotropes of carbon such as diamond, amorphous carbon, carbon nanofoam, or fullerenes, graphene is only one atomic layer thin.
Graphene has an unusual band structure in which conical electron and hole pockets meet only at the K-points of the Brillouin zone in momentum space. The energy of the charge carriers, i.e., electrons or holes, has a linear dependence on the momentum of the carriers. As a consequence, the carriers behave as relativistic Dirac-Fermions with a zero effective mass and are governed by Dirac's equation. Graphene sheets may have a large carrier mobility of greater than 200,000 cm2/V-sec at 4K. Even at 300K, the carrier mobility can be as high as 15,000 cm2/V-sec.
Graphene layers may be grown by solid-state graphitization, i.e., by sublimating silicon atoms from a surface of a silicon carbide crystal, such as the (0001) surface. At about 1,150° C., a complex pattern of surface reconstruction begins to appear at an initial stage of graphitization. Typically, a higher temperature is needed to form a graphene layer. Graphene layers on another material are also known in the art. For example, single or several layers of graphene may be formed on a metal surface, such as copper and nickel, by chemical deposition of carbon atoms from a carbon-rich precursor.
Graphene displays many other advantageous electrical properties such as electronic coherence at near room temperature and quantum interference effects. Ballistic transport properties in small scale structures are also expected in graphene layers.
Despite the fact that the graphene was isolated for the first time only a few years ago (by using ordinary Scotch tape), this field currently being intensively studied. Because of the unique electrical properties such as those described above (e.g., high charge carrier mobility etc.), graphene is ultimately promising for electronic applications. For example, graphene transistors have been demonstrated recently, and more advanced graphene circuits are considered to be promising candidates to perhaps replace silicon in future integrated circuit technology. However, the lack of easy and low cost graphene fabrication processes presently limits the development of graphene applications.